In the integrated circuit industry, there is a trend toward increasing complexity of integrated circuits and circuit elements packed to limit density on substrates and chips. Accordingly, there is also a companion need for more precise printed circuit interconnections. Close dimensional tolerances on the order of fractions of a micrometer require smooth, coherent electrodeposits, typically of the noble metals, of uniform thickness and conformity, particularly free of voids, laminations and protrusions, features that in current practice are hardly if ever achieved.
The metallization or the provision of interconnecting metal lines in semiconductor devices and integrated circuits is currently accomplished using two types of processes. In the first type of process, commonly referred to as additive metallization, a metal, for example a precious metal such as gold, is selectively applied to the desired surface by electroplating in the areas where metallization is desired. In the second type of process, commonly referred as subtractive metallization, a metal, for example aluminum, is applied to the entire area of the desired surface. Various methods are used for applying the aluminum, for example bias sputtering and the like. The desired circuit interconnection pattern is then formed by removing portions of the deposited metal, for example by etching.
Both of these types of metallization processes have advantages and disadvantages related to the configurations of the resulting patterns and their electrical properties. Although precious metals such as gold are more costly than aluminum, in the case of VSLI circuits, the use of plasma etching to form small geometry lines and spaces (having, for example, widths of approximately 2 microns) will substantially increase the cost of aluminum metallization. Moreover, gold interconnects can be formed by electroplating which does not require special etching techniques to obtain anisotropy. Rather, the minimum metal pitch is determined by the resolution capability in the positive photoresist which is employed. Electroplating also provides good step coverage, generally making it unnecessary to taper etch first metal lines or via contacts as is frequently required in processes employing aluminum. However, the additive processes are limited by the ability of the electrodeposits to conform to the photolithogrophy.
A two-layer gold metallization process for bipolar VLSI circuits is disclosed by Summers, Solid State Technology, Dec. 1983, pages 137-141. The Early et al U.S. Pat. No. 4,687,552 similarly discloses the use of the Summers method for integrated circuit metallization. General techniques of electroplating and electrodepositing precious metals and/or precious metal alloys are also disclosed in the Wohlwill U.S. Pat. No. 961,924, the Pokras U.S. Pat. No. 3,505,182, the Dettke et al U.S. Pat. No. 3,749,650, the Okinaka U.S. Pat. No. 4,377,450 and the Abys U.S. Pat. No. 4,478,691.
Electroplating of metals has also been conducted in the presence of high frequency fields. For example, the Hausner U.S. Pat. No. 2,824,830 discloses an electroplating process which involves superimposing on the D.C. field in the electrolyte at least two high frequency fields whose frequencies differ slightly. Hausner discloses that the process provides an extremely dense, finely crystalline metal deposit which is more strongly bonded to its base. The Inoue U.S. Pat. No. 3,503,860 discloses a process for the low-temperature ionic diffusion of a substance deposited on a metallic substrate in an electrolyte. Inoue discloses that the current applied is predominately unidirectional with a super imposed high frequency component having a frequency of 100 kHz to the order of several mHz. A substance is suspended in the electrolyte and migrates to the substrate where the current causes the substance to ionically diffuse into the substrate. Suitable substances include various metal powders.
However, as noted by Bluestone, Chemical Week, Oct. 16, 1985, pages 18 19, there is a continual search for plating methods which deposit metal more selectively and more efficiently while minimizing adverse environmental effects. Of particular interest are plating methods which either use smaller quantities of gold or use substitute materials. While the semiconductor industry has recently been employing more silver in place of gold in plating methods, the circuit board innerconnector industry has not made a similar change. Irregularities which occur in electrodeposits increase the amount of metals, for example gold, which are required. Thus, a significant need exists for electroplating methods which deposit metal with increased selectivity and efficiency.